1. Field of the Invention
The present invention relates to a process for preparing malonic acid and alkylmalonic acids of formula I ##STR2## where R.sup.1 =H, CH.sub.3,
R.sup.2 =H, CH.sub.3 or PA1 R.sup.1 +R.sup.2 =--CH.sub.2 --CH.sub.2 --,
by acid catalyzed saponification of the corresponding methyl, ethyl, propyl, isopropyl or tert-butyl esters.
2. Description of the Background
Malonic and alkylmalonic acids are important intermediates in the synthesis of agrochemicals and pharmaceutically active ingredients. They are used, for example, for the preparation of Meldrum's acids, barbiturates, fragrances and vitamins.
The preparation of malonic acid and alkylmalonic acids is described, in general terms, in Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition (1981), Vol. 14, 794-810. Acid or alkaline saponification of esters, nitrites or amides of carboxylic acids can be used. One problem often encountered in the process is that the products are obtained as mixtures with alkali metal salts, e.g. sodium chloride.
Complicating factors in the preparation of malonic acid and alkylmalonic acids are their excellent solubility in water and their ready decarboxylation, which, in the case of malonic acid, begins at temperatures as low as 70.degree. C.
The best-known preparation of malonic acid and also methylmalonic acid starts from chloroacetic acid and .alpha.-chloropropionic acid, respectively, which are reacted with sodium cyanide to give the corresponding nitriles, which nitriles are then saponified with sodium hydroxide solution, NH.sub.3 being eliminated. According to Organic Syntheses, Vol. II (1943), 376, this procedure is followed by laborious isolation via the Ca salts.
If, after the alkaline saponification of a malonic ester, the malonic acid is present as the alkali metal salt, dissolved in water, the alkali metal cations can be removed, as described in DE 41 20 704, via acid ion exchangers, followed by isolation of the free acid. Since alkaline saponifications always require the use of at least stoichiometric amounts of alkali, which are neutralized again with strong acids in the course of malonic acid work-up, this process produces at least stoichiometric amounts of salt, which often have to be disposed of at considerable expense. This is a major drawback of this method of alkaline saponification.
In acidic saponification of derivatives of malonic acid, inorganic acids are added in catalytic amounts. For example, the saponification of diethyl malonate with sulfuric acid can proceed at 70.degree. C. to give malonic acid, the malonic acid being obtainable from concentrated aqueous solution by crystallization. The drawbacks of the process of acidic saponification are unsatisfactory yields, because of the decomposition of the malonic acids in a strongly acidic environment, and corrosion problems in scaling up of the process.
According to Org. Syntheses, Vol. 60 (1981), 66, cyclopropane-1,1-dicarboxylic acid can be prepared in a yield of about 70% from diethyl malonate, 1,2-dibromoethane, sodium hydroxide solution and stoichiometric amounts of a phase transfer catalyst by simultaneous saponification of the diethyl cyclopropane-1,1-dicarboxylate formed as an intermediate. In addition to the large quantity of phase transfer catalyst, 2.5 times the amount based on the cyclopropane-1,1-dicarboxylic acid obtained, considerable quantities of sodium chloride are also produced which, dissolved in water, require disposal.
Dimethylmalonic acid has likewise been known for a long time and can be prepared by alkaline saponification of the corresponding diethyl ester. Saponification of the dimethyl ester with potassium hydroxide solution to give the acid is mentioned by W. Schauzer, K. Clusius, Z. Physik. Chemie A 190 (1941), 243, without the yield and purity being stated. According to other processes, the acid is obtained by oxidation, employing KMnO.sub.4 or HNO.sub.3, of methyl precursors. A need, therefore, continues to exist for a method of hydrolyzing malonic ester to prepare malonic acid or malonic acid compounds in good yields with less contamination with salts.